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This allows for cases where the request complete callback is invoked outside a message loop,
for e.g. in TestPostChunkedDataAfterStart(). Quit() can only be invoked inside a message
loop whereas posting a QuitTask to the message loop can be done from anywhere.

BUG=none
TEST=TestPostChunkedDataAfterStart passes through the tsan bots and net_unittests in general runs fine.

Review URL: http://codereview.chromium.org/6329019

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@72493 0039d316-1c4b-4281-b951-d872f2087c98